A release valve apparatus of this kind is known, for example, from German Published patent application No. 1 530 242. According to this publication, the release valve is placed in that part of the connection which is between the air reservoir and the brake control valve. The other admission area in this release valve is always connected with atmosphere via a nozzle. The piston has a tappet to actuate the two-way valve, which has two stop valves switchable opposite one another. One of the two stop valves is located in a connection of two line segments discharging into the release valve. One of these line segments links the release valve with the air reservoir and the other line segment links it with the brake control valve. The other stop valve monitors a junction of the line segment leading to the brake control valve through a connecting bore in the tappet to the other admission area. In the rest position of the prior art valve the first stop valve is open and the two line segments are linked with one another, so that the brake control valve can feed air from the air reservoir to the brake cylinder during braking. The second stop valve is closed in this process, and the other admission area is thus under atmospheric pressure. When a vacuum impulse is injected into the main air line and thereby the one admission area, the release valve switches, dividing the two line segments from one another through closing the first stop valve, and, through opening the second stop valve, connecting the other admission area to the line segment leading to the brake control valve and thereby to the brake cylinder via the brake control valve, which is the brake position. Compressed air then streams from the brake cylinder through the second, open stop valve and the connecting bore of the tappet into the other admission area of the release valve and holds the release valve in its operative position through back pressure in this admission area, while at the same time this compressed air flows out of the other admission area via the nozzle to atmosphere. Through the back pressure in the other admission area the release valve is thus held in its operative position until the brake cylinder is almost completely emptied. At the latest when compressed air is again supplied to the main air line, the release valve returns to its off position. There is no assurance, however, that the brake control valve will be in its release position when the release valve returns to its rest position. If the brake control valve should still be in its braking position, compressed air will be supplied from the air reservoir into the brake cylinder and then, when the brake control valve reaches its release position, will be released again. The brief braking process that results can disturb operations and involves an unnecessary energy loss.
It is significant that, when the release valve is actuated, only the brake cylinder but not the air reservoir of the air brake is discharged.
The prior art release valve apparatus described above is suitable for use in graduated air brakes in which at the most one small, constant-pressure control air reservoir is provided in addition to the air reservoir. For direct-release air brakes, which often have an additional emergency brake reservoir in addition to the mentioned air reservoir to increase the pressure that can be injected into the brake cylinder during emergency stops, such release valve apparatus is not suitable. In emergency braking with these direct-release air brakes, the emergency brake reservoir is connected to the brake cylinder via the emergency brake part of the brake control valve, and thus is discharged together with the air reservoir when the release valve is actuated.
While a release valve that can be actuated through a vacuum injected into the main air line and is intended for direct-release air brakes of rail vehicles can be inferred from U.S. Pat. No. 1,765,152, this known release valve operates only through venting of the sole air reservoir provided. This release valve thus, in contrast to the previously described known release valve apparatus, causes a large loss of power through draining the total supply of air of the air brakes, and considerable time is required to release the brakes and subsequently recharge the air brakes.
Also known for direct-release, indirect air brakes of rail vehicles are release valves that are located in the area lying between the brake control valve and the brake cylinder of the connection extending from the air reservoir to the brake cylinder. A release valve of this kind can be inferred, e.g., from U.S. Pat. No. 3,117,823. These release valves, however, can be operated only manually; their piston serves only to keep the release valve open after brief manual actuation of the release valve until the brake cylinder has been completely discharged. Actuation of this known release valve through a vacuum induced in the main air line is not possible.